New wireless technology is being developed and deployed to support voice and multimedia services in both residential and enterprise environments. Wireless Local Area Network (“WLAN”) devices, for example, are being developed in conjunction with IEEE 802.11 standards to support packetized voice communications such as Voice over Internet Protocol (“VoIP”) over Wi-Fi. There are technological hurdles that must be overcome in order to support voice, video, video teleconferencing, remote display, gaming and other multimedia applications on WLANs because the WLAN technology was initially designed to support simple data communications. In particular, voice and multimedia applications can be more sensitive to jitter, delay and packet loss than data communications applications. It is therefore important that wireless devices be tested to verify their ability to support VoIP and other multimedia applications prior to network deployment.
Testing typically involves monitoring, capturing and analyzing traffic that is exchanged between a pair of communication nodes to quantify the performance of the network devices. One characteristic that is used to measure the performance of the network devices is the packet drop rate of the devices. The packet drop rate identifies a percentage of packets that are received with errors or not received at all by the device. Another characteristic of a network node that requires accurate measurement is the delay through the device or network of devices and the transmission medium, and how the delay is affected by environmental conditions. A third characteristic which is measured is the jitter of a system, wherein the jitter is the variation in delays between successive packets. Devices which receive VoIP communications are particularly sensitive to packet loss, delay and jitter, and it is therefore important that the drop rate, delay and jitter of VoIP devices be accurately measured. The packet drop rate, delay and jitter characteristics of a network are directly related to the quality of service that can be provided to voice, video and other multi-media applications. The measured performance characteristics therefore give and indication of the quality of service that can be provided by a system under test.
One problem with accurately determining the drop rate characteristic of a device is that the monitoring devices are also subject to packet loss. When capturing traffic for analysis of network conditions or device performance, the monitor's receiver sometimes receives frames with errors or loses them altogether. Since monitoring equipment uses the same radio or receiver technology as the devices it is monitoring, such loss of frames is a common problem. A monitoring device may fail to detect receipt of a packet that is accurately received by the device under test (DUT), and characterize the packet as a dropped packet erroneously. A monitoring device that erroneously identifies a packet as dropped mischaracterizes the performance of the device under test. It is desirable to increase the accuracy of packet drop rate measurements to minimize mischaracterization of DUT performance.
Packet delay measurement involves identifying the amount of time it takes a packet to propagate or be forwarded between one point in the network and another. One problem with delay measurements, however, is that they do not typically include delays associated with re-transmissions of the packet. The number of retransmissions may vary based on network conditions and how the device reacts to environmental conditions, such as external traffic and noise. The present invention realizes that retransmissions are part of the delay that has to be measured and that an accurate delay measurement should be measured from first transmission to a good packet reception.